Mercado has pleaded not guilty to three counts of first-degree murder. On Wednesday, after two days of evidence was presented in the preliminary hearing, he was bound over for trial.

Flint and Salvatore were fatally shot inside a car in the Mission Valley mall parking lot on Dec. 24, 2013. Gianni’s body was discovered on Jan. 17 inside the trunk of his own vehicle, which was parked 100 miles away in Riverside.

The day after Gianni’s decomposing corpse was found, a U.S. Border Patrol agent pulled Mercado over as he drove his 2001 white Ford Explorer through the Interstate 5 checkpoint near San Clemente, the search warrant says.

The agent told investigators Mercado seemed “dazed” and "drained of emotion." After some questioning, he said he was going to San Bernardino to find a shooting range and told the agent he had an AR-15 in a bag laying on his backseat.

A search of Mercado’s vehicle uncovered a mask, zip ties and the AR-15 assault rifle with three fully loaded, ten-round magazines and two fully loaded, 30-round magazines. They also found one .45 caliber semi-automatic handgun with three, fully loaded magazines and spare barrel, as well as one .22 caliber semi-automatic handgun with two loaded magazines.

A handmade silencer was discovered hidden in the SUV’s center console, the search warrant says.

They finished processing the crime scene where Gianni’s body was discovered, pulling DNA evidence and fingerprints from a can of Febreze air freshener and two boxes of Arm and Hammer air freshener.

The Febreze canister’s trigger was duct taped down so all of the contents would spray into the trunk and mask the odor of Gianni’s body, the SDPD detective says. On the duct tape, analysts found a single black hair.

Analysts also took DNA swabs off the gas cap of Gianni’s car, assuming whoever drove it to Riverside may have had to refuel it along the way.

But when all the evidence was cross referenced with the California DNA Data Bank in March, investigators could find no suspect matches, according to the search warrant.

One month later, they hit a break in the case.

The California DOJ again arrested Mercado outside his Mira Mesa home on April 29. He was charged with possession of an assault weapon, possession of a firearm silencer and carrying a concealed weapon in a vehicle.

Mission Valley Mall Christmas Eve Shooting: Images

During that felony arrest, his fingerprints and DNA samples were entered into the state database.

On June 18, the search warrant says a data bank administrator notified SDPD detectives that they had a match for the triple homicide suspect’s DNA profile: Carlo Mercado.

The California DOJ worked with the SDPD to transfer evidence in Mercado’s case, including his .22 caliber semi-automatic handgun.

The SDPD firearms unit examined the gun and compared its shell casings to casings found inside the vehicle where Salvatore and Flint were killed.

The criminalist believes Mercado’s gun fired the shots at all three victims, a firearms analyst testified at Mercado's preliminary hearing Tuesday.

However, the detective who wrote the search warrant says he is not sure if he believes Mercado acted alone or if he had co-conspirators. He hasn't determined a motive in the killings, either.

Mercado's defense attorney questioned homicide detectives at Tuesday's pre-trial hearing about the victims' drug use suggesting that may have been a factor in the case.

Detective Timothy Norris testified Salvatore had completed a 3-month rehab for heroin addiction, Flint smoked heroin and a drug dealer told police that Gianni had bought heroin from her daily since August 2013. Gianni's alleged heroin habit cost up to $250 a day, Norris testified.

When the search warrant was executed after Mercado's June 20 arrest, the detective hoped to find clothing worn during the slayings, documents that tied Mercado to the victims or their families or other electronic evidence.

Officers seized boxes of ammunition, documents, CDs, computers, clothes, external hard drives and more from Mercado’s Mira Mesa home. They also found the same type of duct tape in Mercado's closet that was used to hold down the Febreze nozzle inside the Riverside trunk, a crime scene specialist testified.